I have included a side and front view of the German Customs ship I am using as a basis for the design.
Well keep in mind that:
- you cannot have your engines, or indeed any machinery or engineering, in the hulls, or at least very little of it. Which means you need it in the superstructure - hence the built-up superstructure
- weapon systems are big - unless you're talking about even up to an OTO 76, but missile launchers especially are space hogs. They're either going to eat up deck space, or in the case of VLS systems, deck space and
hull space. This is why ships of a certain size can only carry certain systems - more likely than not, they've been designed around these systems, and/or designed around a particular performance parameter (nowadays, its usually carrying around a specific system, like say SPY-1D for example, and having long endurance. If you're a nation with carriers, since you have carriers to escort, you're also going to throw in high speed requirements to keep up with them).
- weapon systems are also space hogs indirectly too - by way of the sensors equipment the require, which more often than not are also
serious space hogs themselves, especially any system that's going to be worthwhile. SPY-1, for example, is just friggin' huge - you literally hang the face panels off the superstructure (ala DDG-51). S-1850, a comparable, high-resolution set, is also friggin' huge. You also more often than not need missile directors, so that the missile can use the data it's getting from the radar and make that information useful - such as converting it into a beam of some sort to follow and "ride" on, or by refining the search to a specific area of sky the missile can then know to point at. Even active-homing missiles need some sort of initial guidance upon leaving the launcher, and the actuality of it is that, IIRC, there is no such thing as an active homing missile that has radar with enough range to be "fire and forget" after launch. Also, you need computer rooms and a big enough C&C room to be able to use all of this. These tend to be very large rooms.
- all of this, and all of your ships functions too, require crew. The more functions you add, the more crew you need. Small IPVs and OPVs have simple functions - they more or less scare bad guys away by virtue of being armed with anything really, and that even if the bad guys are armed, almost universally their kit sucks. This is actually why the OHPs are kinda nifty OPVs even though its kit, well, sucks, or at least is horribly inefficient if you're just looking at the raw tonnage. Its saving grace is that it brings a lot of functionality to the OPV mission - it has helicopters which are really
useful for scaring away pirates, it has big facilities for those helicopters, it can deploy and recover RHIBs, it has enough space and accommodations for very well armed and large boarding teams, its physically big enough to be imposing to small craft, it has a clear, distinctive profile that cannot be mistaken for anything but a warship, and it has enough bunkerage to stay on-station for a very long time. Because it's also a "major warship" it can also be used for "showing the flag" missions and making port calls, which again its bunkerage is very important for. It's also 4,000 tons and has a large crew for its size, and is under-armed compared to ships 500-1,000 tons less its size and with significantly smaller crews, and even when it had the Mk 13 that point was still in contention.
The point being, is the classic thing about trade-offs. The way I like to think of it, very few ships are built with trade-offs in mind; rather they are built for specific missions. OHPs were built as ocean escorts, which means they needed long range and needed helicopters, which are bar-none the best weapon to have against a submarine except another submarine. They do this very well. Burkes were built to escort carriers, which means they needed a fast cruise speed as well as long range, and other than that to have the best sensors and weapons the USN had in possession. They do this very well too. So you have to think of what kind of mission you're going for, and make sure it has the proper kit for the mission, and everything else is going to be "traded off" because it doesn't really factor in in the first place.
So now to go over what you did get right - the Millennium Gun is a decent choice for a craft like this - it's got minimal deck penetration and it scares away bad guys because unless the bad guys start buying ex-Soviet frigates there is nothing they're gonna have that is going to outgun a friggin' 35 millimeter autocannon. But four is excessive and if nothing else points to a very bad compromise in terms of weapons placement. Which means you need to go back and redesign the superstructure to ensure efficient arc coverage.
RHIBs are essential for a ship of this nature - but do you have adequate launch facilities, i.e., is your launch system more ideal for sending people off and far away from a sinking vessel, or is it actually ideal for lowering a bunch of men wearing armor vests and carrying a lot of guns? Do you have an adequate recovery system for your RHIB, along with the men that originally went out with it, along with some extra men they might be bringing back for interrogation or holding?
Water cannons are fine and give the ship some useful firefighting capability as well. But some Long-Range Acoustic Devices would be a plus. A lethal solution wouldn't be bad either in the form of some manned 25mm cannons up front - just because it's lethal doesn't mean it has to be used in a lethal situation. Having a person, preferably some big, burly jock who looks like he could be a part of the ship's Marines detachment down to the shaven head, is ridiculously
intimidating, moreso than the unmanned cannon turret. It'll make pirates think twice about firing back, even if the guy in an open gun mount is more vulnerable than an unmanned turret.
Finally, I wouldn't call accommodations non-essential. Crew has to stay somewhere.